The present invention relates to medical ultrasound diagnostic equipment. In particular it relates to Doppler blood velocity meters.
In the measurement of blood velocity using pulsed ultrasound Doppler equipment, there is a recognized problem in measuring the velocity of blood in deep lying vessels. The problem results from the fact that the pulse repetition frequency (hereinafter called "PRF") is determined, in part, by the depth within the body of the blood whose velocity is being measured. The PRF is typically selected such that a pulse can be transmitted from the transducer and reflected from blood flowing within the vessel with the return pulse being received prior to the transmission of the next succeeding pulse.
As used herein the term "sample volume" means the region of interest of blood flow velocity. The terms "sample volume" and "PRF" are well known and understood in the art, and it is also well known that the maximum PRF which can be used without introducing depth ambiguities is equal to the speed of sound in the medium divided by twice the depth of the sample volume.
A well recognized phenomenon, called "aliasing", wherein the blood flow appears to have a different velocity or direction than it actually has, occurs when blood flow exceeds a maximum velocity for a given ultrasound transmitted frequency. Aliasing results from the fact that the Doppler shift frequency is equal to twice the ultrasound transmitted frequency times the velocity of the moving blood divided by the velocity of sound in the body. When the Doppler frequency is more than one half the PRF, aliasing occurs. In other words, aliasing occurs when the maximum blood flow velocity is greater than or equal to the square of the speed of sound in the human body divided by eight times the ultrasound transmitted frequency times the sample volume depth. Accordingly, the maximum blood flow velocity which can be measured without exhibiting aliasing is inversely proportional to the sample volume depth in the body for a given ultrasound transmitted frequency.
While one approach to increasing the maximum velocity which can be measured without aliasing is to reduce the frequency of the ultrasound transmitted energy, if the frequency decreases below about 2 MHz, the scattering phenomenon, which is required for observing the return Doppler signals, is degraded. In addition, a reduction in the ultrasound transmitted frequency also reduces the resolution of the sample volume. Accordingly, the approach of decreasing the frequency of the ultrasound transmitted frequency can be helpful to about 2 MHz. Thereafter, decreasing the ultrasound transmitted frequency has not been found to be a desirable approach to use for eliminating the aliasing effect. Accordingly, a new approach to providing an unaliased signal would be desirable.
During normal operation of a pulsed Doppler system, a new burst is not transmitted until the return from the location of interest, i.e. the sample volume, is received. If the velocity of sound in body tissue is c, then the time of flight, T, from the transducer to the sample volume depth, d, and back is: EQU T=2d/c
Thus, the highest PRF that is normally used is: EQU PRF.sub.max =c/2d
The detected Doppler shifted frequency, f.sub.d, of a target moving with velocity, v, detected with carrier frequency, f.sub.0, is given by: EQU f.sub.d =2f.sub.0 v/c
As PRF/2 is the highest frequency that can be measured without aliasing, EQU f.sub.d =PRF/2=2f.sub.0 U.sub.max /c
Accordingly, the highest velocity, U.sub.max, which can be unambiguously detected is: EQU U.sub.max =PRF*c/4f.sub.0 =(c.sup.2)/(8f.sub.0 d)
Heretofore, to prevent the problem of aliasing, either continuous wave Doppler was used, and all range resolution was lost, or, alternatively, the transmitted frequency was decreased. As noted above, decreasing the transmitted frequency only works down to a frequency of about 2 MHz due to decreased scattering. Increasing the PRF over PRF.sub.max introduces range ambiguities, and tracking the mean frequency as a function of time requires knowledge that the signal is not aliased at some point in time as a reference and the knowledge that it does not change too rapidly. (See "Resolution of Frequency Aliases in Ultrasound Pulsed Doppler Velocimeters", Craig Hartley, IEEE Trans. Sonics and Ultrasonics, Vol. SU-28, 1981, pp 69-75.). All of the foregoing approaches sacrifice some significant aspect of pulsed Dopplers, such as range resolution, or, alternatively, they require that some assumptions be made which may not be valid.